I find Narcissa far and away the most interesting of the Black sisters - Bellatrix is simply insane, Andromeda boringly normal, Narcissa is more complex as you say, able to judge situations intelligently, unflinchingly change her mind and determined to protect what she sees as important, usually her family - and who could blame her for that? That's what Lily and James did, after all!
Unlike her husband (bless his silken socks) I can see her weighing up situations and considering what is the right thing to do, as well as the personally advantageous choice, and then deciding how to make that choice work best for her. I rather like her. Heh. I just like Slytherins.
Thank you for your comment - and your comma-taming skillz!